The molding of polyurethane foam articles is well known. A wide variety of molding processes and foam-producing components are known to those skilled in the art. It is common practice to fill an open or closed mold with premixed polyol and isocyanate components through a feed channel. The exact nature of the foam-producing components depends upon the desired physical characteristics of the final product and whether working is to take place in accordance with a single stage (direct) process or a two-stage (prepolymer or semi-prepolymer) process. Such processes are described in detail in the literature and reference is made to "Integralschaumstoffe", Piechota and Rohr, 1975; Kunststoff - Handbuch, Volume VII, "Polyurethane", 1966; "Schaumkunststoffe", published by the Fachverband Schaumkunststoffe in GKV, 1976; and Wittfoht, Kunststoff - Technisches Worterbuch, part 3.
To permit easy and clean removal of polymerized foam articles from metal, wood or plastic molds, the mold surfaces which come into contact with the foam-producing components or the cured polyurethane foam are treated with release agents prior to filling the mold. To produce a uniform film on the mold surfaces, release agents are applied by known spraying and injection processes wherein the release agent is atomized either under the influence of a pressurized gas (e.g. air), or in an "airless" system wherein the release agent is pumped under high pressure through an atomizing nozzle. In addition to providing good mold release characteristics, the release agent often is called upon to influence the surface characteristics of the finished polyurethane foam article. In these cases, e.g. in the cold flexible mold field (seats, headrests and armrests of cars, etc.), certain additives produce the open-cell characteristics and consequently the required "breathability" of such foam articles.
Conventional release agents consist of organic solvents and release-active substances dissolved, dispersed, suspended or emulsified therein and which are normally considered "solids". As is well known, release agents can contain waxes, fats, silicone compounds, plasticizers, stabilizers, accelerators, etc. The solvents (such as methylene chloride, trichloroethane, perchloroethylene, petroleum and high-boiling petroleum hydrocarbons and refrigerants) serve as carriers, so that the release-active substances can be applied in a uniform film to the mold surfaces.
The particular release-active substances, amounts thereof and solvent(s) employed in a given release agent are chosen with respect to the mold temperature, the airing time (time between release agent application and prepolymer introduction) and the particular foam system (integral, rigid integral, cold flexible, etc.) employed. Thus, a wide variety of articles with widely varying characteristics are produced by those skilled in the art. Integral foam is used for producing automobile steering wheels where uniformity and good gripping characteristics are desired. When producing integral foam, the mold temperature is generally between 30.degree. C. and 50.degree. C. RIM process foam, which is frequently used for producing polished moldings, requires a mold temperature of about 50.degree. C. to 70.degree. C. and, consequently, a different release agent. Cold flexible foam articles, such as seats, headrests and armrests for cars, etc., are produced at mold temperatures in the range of 40.degree. C. to 70.degree. C. Mold temperature is generally 30.degree. C. to 50.degree. C. when producing a rigid foam articles, such as brackets for cars, window shutters, refrigerator components and furniture.
Upon introducing the release agent to the heated mold surface, the solvents evaporate as an azeotropic mixture. These solvents, together with the overspray, must be removed from the work area. Such removal operations result in the loss of valuable raw materials as well as the emission of pollutants which are highly prejudicial to the environment. Recycling processes via adsorption (to reduce harmful emissions) are conceivable, but would be greatly hindered by the "solid" release-active substances which are present in finely divided form.
As a result of the aforementioned problems, attempts have been made to use release agent concentrates with low solvent proportions, as well as release agents having an aqueous base, i.e., release agents in which organic solvents are largely and preferably totally replaced by water. The spraying of release agent concentrates is difficult because of the high viscosity of such concentrates. The use of aqueous release agents has proven problematical because the foam-producing components cannot be introduced into the mold until the solvent (water) has evaporated. The "airing times", or portions of the manufacturing cycle devoted to the evaporation of the release agent solvent, are in general between 20 and 60 seconds when an aqueous release agent is used. Such extended airing times unduly delay the manufacturing process and render the use of aqueous release agents uneconomical. Organic solvents allow the use of much shorter airing times, but such shortened times cause considerable problems when used in connection with aqueous release agents.
It is known that aqueous release agents have previously been used in a few fields (hot foam, modified integral foam in back-foaming processes). These aqueous release agents are wax dispersions or emulsions whose use has been subject to serious limitations regarding the surface characteristics of the foamed articles. Further, in the aforementioned foam production processes and at the mold temperatures used therein, considerable airing times are required so that the use thereof in industrial mass production is not economically feasible.
As previously mentioned, the incomplete evaporation of water from an aqueous release agent leads to lower quality foamed products. This is primarily due to a competing reaction, running parallel to the polyolisocyanate reaction, between water and isocyanate groups (R--N.dbd.C.dbd.O+H.sub.2 O.fwdarw.R--NH.sub.2 +CO.sub.2), which liberates carbon dioxide and which partly displaces the precisely matched polyol: isocyanate ratio. This leads to foam problems, such as discolorations, bubbles, blisters and even partial foam collapse.
To avoid these disadvantages which result from the incomplete evaporation of aqueous release agents, release agents have been used which, in addition to water, contain considerable proportions of low boiling water soluble alcohols, ketones, esters, etc. as evaporation accelerators. Although these components lead to a certain reduction in the excessive evaporation times of aqueous release agents, such reduction is not sufficient to avoid the aforementioned reaction between the water and isocyanate. Moreover, and as stated earlier, these water soluble evaporation acclerators are very prejudicial to the environment. As a result of the aforementioned disadvantages, such aqueous release agents, which generally contain considerable solublizer proportions, have not been adopted in practice.
Accordingly, it is an object of the present invention to provide a process for producing polyurethane foam articles which permits the use of aqueous release agents, or release agent concentrates containing minor proportions of organic solvents, to thereby considerably reduce or avoid the prejudice to the environment linked to the use of organic solvents.
Another object of the present invention is to provide a process for producing polyurethane foam articles which obviates the disadvantages linked to the poor evaporation behavior of aqueous release agents and the high viscosity of release agent concentrates containing organic solvents.
These and other objects are achieved according to the present process wherein an aqueous release agent or a release agent concentrate containing relatively minor amounts of organic solvent is preheated prior to being applied to a mold in per se conventional manner, e.g. by atomization. The release agent is preheated in a variety of ways, such as by warming the liquid release agent before it reaches the atomizing nozzle and/or by atomizing the release agent with the aid of hot, pressurized gases, particularly hot compressed air.
As mentioned earlier, the present process is practiced by injecting or spraying a release agent into a mold, the release agent being atomized either under the influence of a compressed gas, such as compressed air, or via an "airless" system wherein the release agent is pumped at a high pressure through an atomizing nozzle. Particularly suitable for performing the present process are atomization units which combine the airless and compressed air principle. Such units include an external mixing head or sprayers wherein the atomization of the material takes place by pressure atomization and compressed air atomization.
In an atomization device, the nozzle shape, size, the material (release agent) pressure and the air pressure are chosen as a function of the viscosity of the agent to be sprayed. Guide values for the nozzle diameter are are 0.3 to 1 mm, for the material pressure 0.5 to 1 bar and for the aire pressure 2 to 4 bar, in order to be able to use release agents with viscosities between about 50 and 1,000 cps.